Conveyor tracks are widely used in a great variety of different situations, which generally involve moving articles from one location to another such as moving freight material for example in the freight compartment or hold of a freight aircraft, for the movement of freight containers when loading and unloading the aircraft. One form of such a conveyor track comprises a plurality of rotatable balls on which the freight containers can roll along the track. Such support ball units are subject to a number of requirements, in particular that they are substantially maintenance-free and very robust, while nonetheless being of comparatively light weight.
As is known from U.S. Pat. No. 3,739,894, one form of a conveyor ball unit for the movement of freight on a conveyor track comprises a generally hollow-cylindrical housing which has a vertical center axis, with a conveyor ball which projects upwardly beyond the upward open end of the housing and which is mounted rotatably in concentric relationship with the center axis on a plurality of mounting balls in a mounting shell member. A ball guide means embraces the conveyor ball in an annular configuration from above, to secure the conveyor ball and the mounting balls movably in the mounting shell member. A spring means which bears against the housing resiliently biases the mounting shell member upwardly from below, with the conveyor ball, the mounting balls and the mounting shell member being movable as a unit within the housing. That unit can be urged downwardly along the line of the center axis of the arrangement, against the force of the above-mentioned spring means. The housing itself is releasably fixed to a panel or bed portion of the conveyor track in which the conveyor ball unit is used. In that construction, the spring means is in the form of a coil spring disposed between the bottom wall portion and the rim of the mounting unit in the region of the peripheral edge of the mounting shell member, to urge the mounting unit upwardly against an annular abutment shoulder in the housing when the ball is in the non-loaded condition, whereby the conveyor ball thus projects upwardly beyond the surface of the panel or bed portion of the conveyor track, through the central opening in the arrangement. When the ball is subjected to a loading by a freight container which is carried thereon, the conveyor ball together with the entire mounting assembly therefor can be urged downwardly into the housing, against the force of the coil spring, until the underneath surface of the mounting shell member comes into contact with a suitable abutment formed in the housing.
It has been found in practice however that that construction gives rise to the difficulty that the conveyor ball is frequently subjected to a loading not only by purely vertically downwardly directed forces but in general also by lateral thrust forces which have to be carried by the rim portion of the upper opening of the ball guide arrangement. The ball guide arrangement must therefore be of a very strong construction and must be a very accurate fit around the conveyor ball. In addition, when the conveyor ball is subjected to a loading by a lateral force component, it frequently happens that the mounting assembly is urged downwardly in a tilted condition in the housing as the coil spring, by virtue of its very nature, cannot withstand lateral forces but easily deflects under the influence thereof. If, from a tilted position of that nature, the mounting unit is urged upwardly in the housing again by the force of the coil spring, it may then happen that the upper annular boundary flange of the ball guide arrangement may strike against the underside of an annular shoulder formed at the upper edge of the opening of the housing so that the mounting unit and the conveyor ball can no longer return to their normal rest or inoperative position, that is to say the position that the components assume when the conveyor ball is not subjected to the weight of a freight container or the like thereon. In that case the conveyor ball unit jams and has to be restored to its normal operating position manually by operator intervention. In such a situation the conveyor ball unit may also suffer from damage.
Furthermore, the above-discussed conveyor ball unit is made entirely of metal so that it tends to be of substantial weight. Furthermore, because of the necessarily accurate fit between the ball guide arrangement and the conveyor ball itself, manufacture of that conveyor ball unit requires a large amount of machining and just slight amounts of damage to the ball guide arrangement under the rough conditions of use which are frequently encountered in a freight loading and unloading situation may result in a high level of resistance to satisfactory movement of the ball so that the ball is unable to perform its load-supporting and moving function satisfactorily.